Consolidation of nanocrystalline Fe-1.6 wt%C via low temperature hot isostatic pressing

NatuShu~

Pergamon

Mataiab, Vol. 10,No. 1.pp. 3543.1998 Elsevia ScieaccLtd 0 1998Ach Metallurgica Inc. PrintedintheUSA. All rightsre?uvd 0965-9773/w $19.00+ .OO

PII SO9659773(98)00028-2

CONSOLIDATION OF NANOCRYSTALLINE Fe-l.6 wt%C VIA LOW TEMPERATURE HOT ISOSTATIC PRESSING TM Lillo and G.E. Korth Lockheed Martin Idaho Technologies, P.O. Box 1625, Idaho Falls, ID 834152218 (AcceptedApril 19,1997) Abstra’ct-Ball-milled Fe-l.6 wt% C powder with a nanocrystallinesubstructure was consolidated to full density via hot isostaticpressing at 500°C and 414 MPa. The HIPped microstructu)reremained largely nanocrystalline(-39 nm in diameter). Although the grain size increased, thegrain sizedistributionwasnotsignificantlyaltered. The resultsof thisstudysuggest that other rranocrystallinematerials in powder form may be consolidated via HIPping at temperatureslow enough toprevent extensivegrain growth. 01998 Acta MetallurgicaInc. INTRODUCTION Previous work on a variety of materials suggests that various extrinsic material properties are significantly different when the grain size falls within the nanocrystalline regime (grain size is less than 100 nm). Properties affected by nanocrystalline grain size include strength (1-3) and hardness (4,5), which increase, as well as electrical (6.7) and magnetic (89) properties. Furthermore, formability (10) at elevated temperatures is increased, with superplastic behavior (11) being observed in some cases. Unfortunately, producing industrially useful materials with microstructures on a nanocrystalline scale is not trivial. High energy ball milling of metallic powders is one of the more promising methods for commercial production of nanocrystalline materials and typically produces particles, usually >1 pm, with nanosized grains (less than 100 nm). However, consolidation of this powder into fully dense monolithic material must not significantly affect the nanocrystalline substructure if the altered properties are to be retained. Recent studies (12,13) show that nanocrystalline Fe- 1.6 wt% C can be consolidated by dynamic compaction without adversely affecting tbe nanocrystalline microstructure. However, monolithic material produced by this method often suffers from defects (13) such as microcracks and, in the case of explosive compaction in cylindrical geometries, a “ma&” stem area. (The math stem is a somewhat porous region at the center of cylindrical samples where bulk melting occurs due to high rates of deformation and compaction pressures exceeding 40 GPa.) However, compressive deformation of dynamically consolidated samples (13), even ones containing such defects, suggests monolithic nanocrystalline Fe-l.6 wt% C possesses extensive plasticity at temperatures as low as 500°C (T = 0.55Tmp) with virtually no strain hardening. Furthermore, 35

36

TM LILLOAND GE KORTH

static anneals (1213) at temperatures approaching 550°C for one hour produce relatively minor increases in the grain size, indicating that the microstructure is relatively stable at this temperature in the absence of applied stress. The metastable grain size and extensive plasticity at 500°C suggest that it may be possible to consolidate nanocrystalline Fe-l.6 wt%C powders to full density, free of defects, via hot isostatic pressing at relatively low temperatures. This paperreports initial work on the characterization of nanocrystalline monolithic Fe- 1.6 wt%C produced by hot isostatic pressing at 500°C. Comparisons were made to monolithic material produced by dynamic consolidation. EXPERIMENTAL

PROCEDURE

A mixture of commercially pure iron and graphite powder, in portions to produce material of nominal composition Fe-2.0 wt%C, was ball-milled in an attritor (Union Process Inc., Model SDI) operating at 300 rpm for 75 h under flowing argon to reduce oxygen and nitrogen contamination. The ball-to-powder ratio was approximately 20: l(5 mm diameter 440C stainless steel balls). The carbon content of the resulting nanocrystalline powder was approximately 1.56 wtl, with contamination levels of nitrogen and oxygen on the order of 0.34 wt% and 3.9 wt%, respectively (13). The size of the individual powder particles was on the order of 5-10 l.un. For consolidation, the ball-milled powder was loaded into 12.7 mm diameter stainless steel cans, which were then evacuated and sealed. (The powder was not processed to reduce the levels of oxygen and nitrogen contamination prior to being sealed in the evacuated HIP canisters and, therefore, the HIP consolidated material can be expected to retain relatively high contamination levels of these elements;) Hot isostatic pressing was performed in the Quick HIP facility at the Institute of Materials Pmcessing, Michigan Technological University. The relevant temperatnre, pressure and time schedule is shown in Figure 1. The maximum temperature and pressureattained during I-III’consolidation were 500°C and 414 MPa (-60 ksi). Dynamically consolidated material was produced by the method schematically illustrated in Figure 2. As the detonation front propagates along the driver tube the ball-milled powder is consolidated by pressures in excess of 12 GPa. Further details on the dynamic consolidation process and the defects it generates can be found elsewhere (13-16). Transmission electron microscopy (TBM) was performed on the as-milled powder as well as the HIPped material to determine the average grain size and the grain size distribution. Specimens from the HIPped and dynamically consolidated materials were core-drilled from sections thinned to less than 0.25 mm by mechanical grinding. The as-milled powder was suspended in a thin film of G-l epoxy (GATAN, Inc.) and cured at 75’C for 3 h after which 3 mm diameter discs were produced by punching. All samples were dimpled to less than 100 ~t.musing a South Bay Technology Precision Dimpling Instrument, Model 515, and subsequently ion milled to perforation in a stage cooled by liquid nitrogen on a GATAN DuomiIl operated at 6 kV and a milling angle of 20”. Sample characterization was carriedout in aPhilipsEM420 operating at 120 kV. RESULTS AND DISCUSSION The HlPping schedule shown in Figure 1 produced a highly consolidated rod free of microcracks and the “ma&” stem area typically associated with dynamically consolidated

CONSOLIDATON OFFe-l .6 WT%CVIALowTEMPERATURE Ho-rISOSTATK: PRESSINQ

500

V

I

1’1’1’1’

Pressure 0 Temperature

I700

’

-I

-

600

-

G 500 *

400 d B

300

37

i

- 400 i 5 - 300 E : - 200 fi 2 - 100

0

20

40

60

80

100

120

Time, (min.) Figure 1. A chronological presentation of the temperatures and pressures experienced by the as-milled powder during hot isostatic pressing. Container tube

Figure 2. Schematic illustration of the dynamic (explosive) consolidation process.

38

TM IJUOANDGE KORTH

Figure 3. Optical photomicrograph showing the microstructure after HIP consolidation.

material with a cylindrical geometry. No residual porosity was observed in the HIPped material at either the optical level or the microscopic level (TEM), suggesting full density had been achieved. Figure 3 shows an optical micrograph of the HIPconsolidated material. There are small “islands” of material that differ in appearance from the relatively featureless surrounding matrix. Acursory examination of the local chemistry using energy dispersive x-ray analysis failed to reveal the presence of impurities unique to the “islands” or the matrix. Scanning electron microscopy indicated that the “islands” are composed of numerous grains on the order of 500 to 5000 nm in diameter. Microhardness measurements showed the “islands” to be much softer than the matrix, 200 DPH for the “islands” versus 1170 DPH for the matrix. The “islands” were, therefore, concluded to be areas where recrystallization or rapid grain growth had occurred. Unfortunately, the “islands” are a significant fraction of the HIPped material (-10% by atea in Figure 3). Dynamically consolidated materials have a much smaller proportion of such “islands” (< 1% by area). The cause of the “islands” is, as yet, unknown. However, the wide disparity in hardness suggests that the mechanical properties of the HIPped material should be controlled by the material in the featureless regions. In this preliminary study only one combination of time, temperature, and pressure was investigated Other combinations, especially lower temperatures and/or shorter consolidation times, may prevent the formation of the ‘6islands”in the HIPped material. Transmission electron microscopy showed the relatively featureless areas in Figure 3 to be composed of nanometer-sized grains. Figure 4a is a dark field (g=<2oo>Fe) photomicrograph

CONSOLDATW OFFe-l .6 wr”/C VIALowTEWERANRE HOTISOSTAIK PRESSING

39

Figure 4. Dark field ‘EM photomicrographs with associated diffraction patterns of the (a) HIPconsolidated material, (b) as-milled powder, (c) dynamically-consolidated material, and (d) dynamically consolidated material that had been heat treated for one hour at 500°C.

40

TM LILLO AND

GE KORTH

showing the grain structure of the HIPped material. For comparison purposes, dark field photomicrographs of the as-milled powder and of dynamically consolidated material without and with a static anneal at 500°C for one hour are also shown in Figure 4. The diffraction pattern associated with each photomicrograph in Figure 4 indicates that the grains are randomly oriented and that consolidation and post-consolidation annealing do not lead to the development of a preferred texture. The diffraction patterns for the HIPped and heat treated material contain diffraction rings not present in the diffraction patterns of the as-milled and dynamically consolidated materials. These extra diffraction rings in samples that experienced elevated temperatures are expected to arise from the precipitation of iron oxide and one or more of the various forms of iron carbide, considering the relatively high levels of oxygen contamination (-3.9 wt%) and carbon content (-1.6 wt%) in the original powder. Identification of the actual phase(s) giving rise to the extra rings was found to be intractable due to near-coincidence of diffraction peak positions for the various oxide and carbide phases. Also, further complications in phase identification were produced by peak broadening, resulting from the nanocrystalline size of the phase(s), (TEM observations revealed no microconstituent that exceeded the nanocrystalline scale except the “islands” of material which were identified as alpha-Fe), and the relatively low volume fraction of the phase(s), as concluded from dark field TEM observations using one or more of the extra rings. The dark field photomicrographs of Figure 4 indicate that the various processing schemes have a significant effect on the grain size of the material. The average grain size for each material, as determined from photographic negatives, is listed in Table 1. Typically, more than 150 individual grains, taken from as many different areas on the TFM samples as possible, were used in the calculation of average grain size for each material. The grain sizes within the “islands” were not included in the calculation of the average grain size or the grain size distribution. Table I indicates significant grain growth occurred during HlPping, although the grain size is still well within the range considered to be “nanocrystalline”, i.e. less than 50 nm. While Table I indicates that dynamic compaction is capableof producing material with a much smaller grain size, dynamic compaction also produces numerous defects (13) not found in the HIPped material. The lack of defects and the more conventional processing method make HIPping at low temperatures and moderate pressures the more viable method of producing large parts with a nanocrystalline microstructure. The data used to calculate the average grain size were also used to determine the grain size distributions shown in Figure 5. These plots indicate that all processing methods produced a log-normal distribution ( 17) of grain sizes, i.e. the distributions are random. The relative spread in grain size (17) is related to the inverse of the slope for each plot. Okazaki and Conrad (18) and Rhines and Patterson (17) have shown that the relative spread in grain size remains constant during grain growth, only the average grain size increases during normal grain growth. The plot for the HIPped material exhibits a slope very close to that of the as-processed powder, in agreement with the grain growth behavior observed by Okazaki and Conrad (18) and others (17) during static anneals involving materials with a much larger initial grain size. The slopes of the dynamically consolidated material and the dynamically consolidated plus annealed material are similar to each other but significantly different from those of the as-milled powder and HIPped material The greater slopes of the dynamically consolidated materials indicate smaller relative spreads in grain size. The reason for such a difference is unclear; however, it is possible that further grain refinement occurs during the dynamic consolidation process as a result of the harsh conditions

CONSOUDATKMOF Fe-l .6 W/C

VIALowTEMPERATUREHOT ISOSTATK:PREWG

41

TABLE 1 Avenge Grain Size Proce:ssing Scheme

Average Grain Size, nm

Standard Deviation, nrn

As-Processed Powder

9.8

5.2

Dynamically Consolidated

7.3

2.4

Dynamically Consolidated andlhrat 500°C

24.4

10.7

HlPped 1 hr at 5OWC

39.6

18.1

99.9

I

Fe- 1.6C 0 Powder V Dynamic

99

90

Compacted

I V

t

10

Grain

Diameter,

nm

Figure 5. Grain size distribution for the various processing schemes used in this work. Grain diameter iis plotted as the percentage of grains smaller than or equal to a given grain size.

42

TM LILLOANDGE KORTH

(high strain rates and pressures). However, the important conclusion to be drawn from Figure 5 is that consolidation by HIPping does not change the relative spread in the gram size distribution over that of the as-milled powder and that the increase in the average grain size can be attributed to normal grain growth. The relatively small effect of low temperature HP consolidation on the microstructure is important since extensive grain growth can occur at the temperatures typically used in conventional HIPconsolidation (14). This study shows that it is possible to fully consolidate an Fe-C alloy at temperatures much lower than those considered normal and still retain a microstructure that is mostly nanocrystalline. The large grain boundary area in nanocrystalline materials and the high diffusivity paths provided by grain boundaries are thought to combine to redistribute material necessary to fully accommodate the strains that develop during consolidation and deformation processing (819). Jam and Christman (20) have shown that a nanocrystalline Fe-28Al-Q alloy can be deformed extensively in compression at room temperature without evidence of failure, e.g. micro- or macro cracking. The consolidation behavior of the nanocrystalline material highlighted in this work, as well as the reported deformation characteristics of this and other nanocrystalline materials (20-22), suggest that it may be possible to fabricate monolithic specimens of other, and perhaps most all, nanocrystalline materials via low temperature hot isostatic pressing at moderate pressures for short times. Fabrication of large specimens with ananocrystalline microstructure by relatively conventional methods will not only facilitate investigation of the unique properties of ultra-fine grained materials, but may ultimately lead to the commercial use of nanocrystalline materials.

SUMMARY Monolithic Fe-l.6 wt% C with a nanocrystalline microstructure was produced by consolidation of ball-milled powder with a nanocrystalline substructure via low temperature (500°C) hot isostatic pressing at moderate pressures (414 MEa). The HIP consolidation process increased the average grain size by a factor of four but kept a majority of the material within the nanocrystalline regime. Although the grain size was -5.5 times larger than that of dynamically consolidated material, the HPped material did not have the defects typically foundin dynamically consolidated materials. Successful consolidation of nanocrystalline material by a relatively conventional HJPping process provides hope for the commercial use of nanocrystalline materials in components requiring the unique properties offered by ultra-fine grained materials. However, further work is needed to determine whether HIPping parameters can be found that reduce or totally eliminate the large-grained “islands” found in the HIPped material of this study.

ACKNOWLEDGMENTS The as-milled powder for this work was graciously provided by Dr. J.C. Rawers. The work describedinthispaperwassup~dbytheInteriorDepartment’sBuFeauofMinesunderContract No. JO134035 through DOE Idaho Operations Of& Contract No. DE-ACO794ID 13223.

CONSWMTDNOF

Fe-1.6vn%CvwLow TEMPEFUTUREHOTIS~STATK;PL~ESSINO

43

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22.

Nieman, G.W., Weertman,J.R. and Siegel, R.W., Scrip&~Metallurgica, 1989.23.1679. Lehocxky, S.L., Journal of Applied Physics, 1978,49,5479. Merz, M.D. and Dahlgren,S.D., Journal of Applied Physics, 1975,46,3235. Hofler, H.J. and Averback, R.S., Scripta Metallurgica et Materialia, 1990,24,2401. Lu, K., Wei, W.D. andWang, J.T., Scripta Metallurgica et Materialia, 1990,24,23 19. Lee, P.J.aud Larbalesteir,D.C., Acta Metallurgica, 1987,35,2523. Nimtz, G., Marquardt,P. aud Gleiter,H., Journal of Crystal Growth, 1988,86,66. Bininger, R., Materials Science and Engineering, 1989,A117,33. Schaefer,H.E.,Wurschum,R.,Birringer,R.andGleiter,H.,JournalofLessCommonMetals, 1988, 140, 16l. Karch, J. and Bit-ringer,R., Ceramics International, 1990, 16,291. Mayo, hd.J.,Siegel, R.W., Narayanasamy, A. andNix, W.D., Journal of Materials Research, 1990, 5,1073. Rawers, J.C., Journal of Materials Synthesis and Ptvcessing, 1995,3,69. Korth,G.E.,Lillo,T.M.andRawers, J.C.,inSynthesisandPmcessingofNanocrystallinePowders, ed. D.L. Bourell, TMS, Watrendale,PA, 1996, p. 263. Korth, G.E. and Williamson, R.L., Metallugical Transactions A, 1995,26A, 2571. Wright, R.N., Korth, G.E. and Flii, J.E.,Metallurgical Transactions A, 1989,20A, 2449. Korth, G.E., Flinn,J.E. and Green, R.C. in Metallurgical Applicationsof Shock-Wave and HighStrain-Rate Phenomena, eds. L.E. Murr, K.P. Staudhannnerand M.A. Meyers, Marcel Dekker, Inc., New York, NY, 1986, p. 129. Rhines, EN. and Patterson,B.R., Metallurgical Transactions A, 1982,13A, 985. Okazaki, K. and Conrad, H., Transactions Japanese Institute of Metals, 1972,13,198. Chokshk,A.H., Rosen, A., Karch, J. and Gleiter, H., Scripta Metallurgica, 1989,23, 1679. Jain,M. and Christman,T., Acta Metallurgica et Materialia, 1994,42, 1901. Karch, J., Bininger, R. and Gleiter, H., Nature, 1987,330,556. Hahn, El. and Averback, R.S., Nanostructured Materials, 1992.1.95. -